Also the Illustris Simulation was used to analyse the properties of the FGs in the mass regime \(10^{13} - 10^{13.5} M_{\odot }\)  [1]}. The authors found that the magnitude gap of FGs identified at \(z=0\)  were on average created about 3 Gyr ago. In addition, the fossil central galaxies became more massive than non-fossil ones. This difference was explained as due to differences in the mass acquired through mergers between \(z = 0.1 - 1\) , as can be seen in the left panel of Fig. REF . Fossil BCGs have also a larger number of major mergers than non-fossil ones (right panel of Fig. REF ). Indeed, the last major merger of fossil BCGs was later (e.g. at lower redshift). No differences were found in the distribution of the time formation (\(z_{50}\) ) of fossil and non-fossil halos. The group mass assembly of fossils and non-fossils differs only in the recent group accretion history, in particular in the formation time of the 80\(\%\)  of the mass of the halo. However, semi-analytical models studied the dynamical evolution of galaxies in groups with different formation epochs [2]}. They found that BCGs of dynamically-young groups suffered the last major galaxy merger \(\sim 2\)  Gyr more recently than their counterparts in dynamically old groups and that FGs are somewhere in the middle between the other two populations. However, these authors found a lack of recent major mergers in FGs, that is in agreement with the evolution of their old systems.
